The present invention relates to capacitive pressure transducers. More specifically, the present invention relates to an improved apparatus for mounting an electrode in a capacitive pressure transducer.
FIG. 1A shows a partially sectional side view of an assembled prior art capacitive pressure transducer assembly 100. FIG. 1B shows an exploded partially sectional side view of transducer assembly 100. For convenience of illustration, FIGS. 1A and 1B, as well as other figures in the present disclosure, are not drawn to scale. Also, shading has been used to facilitate understanding of the drawings, however, shading has not been used to indicate the materials from which various components illustrated in the drawings are manufactured.
Briefly, transducer assembly 100 includes a body that defines an interior cavity. A relatively thin, flexible, conductive, diaphragm 120 divides the interior cavity into a first sealed interior chamber 110 and a second sealed interior chamber 112. As will be discussed in greater detail below, diaphragm 120 is mounted so that it flexes, moves, or deforms, in response to pressure differentials in chambers 110 and 112. Transducer assembly 100 provides a parameter that is indicative of the amount of diaphragm flexure and this parameter is therefore indirectly indicative of the differential pressure between chambers 110 and 112. The parameter provided by transducer assembly 100 indicative of the differential pressure is the electrical capacitance between diaphragm 120 and one or more conductors disposed on a ceramic electrode 130.
Transducer assembly 100 includes a metallic housing 140, a metallic top cover 150, and a metallic bottom cover 160. Housing 140, upper cover 150, and diaphragm 120 cooperate to define the first interior sealed chamber 110. Diaphragm 120 defines the bottom, housing 140 defines the sidewalls, and upper cover 150 defines the top of chamber 110. The upper cover 150 includes a metallic pressure tube 152 that defines a central aperture 153. Aperture 153 provides an inlet or entry way into chamber 110. As will be discussed below, electrode 130 is housed in chamber 110 and electrode 130 does not prevent the pressure of a fluid in inlet 153 from being communicated, or applied, to the upper surface of diaphragm 120.
The lower cover 160 and the diaphragm 120 cooperate to define the second interior sealed chamber 112. Diaphragm 120 defines the top, and lower cover 160 defines the bottom, of chamber 112. The lower cover 160 also includes a metallic pressure tube 162 that defines a central aperture 163. Aperture 163 provides an inlet or entry way into chamber 112. The pressure of fluid in aperture 163 is communicated to the lower surface of diaphragm 120.
Electrode 130 includes two metallic conductors 131, 132 disposed on a lower surface 133 of the electrode 130. Conductor 131 and diaphragm 120 form parallel plates of a first capacitor Cl. Similarly, conductor 132 and diaphragm 120 form parallel plates of a second capacitor C2. As is well known, C=A.epsilon..sub.r .epsilon..sub.0 /d, where C is the capacitance between two parallel plates, A is the common area between the plates, .epsilon..sub.0 is the permittivity of a vacuum, .epsilon..sub.r is the relative permittivity of the material separating the plates (.epsilon..sub.r =1 for vacuum), and d is the axial distance between the plates (i.e., the distance between the plates measured along an axis normal to the plates). So, the capacitance provided by capacitors C1 and C2 is a function of the axial distance between diaphragm 120 and conductors 131 and 132, respectively. As the diaphragm 120 moves or flexes up and down, in response to changes in the pressure differential between chambers 110 and 112, the capacitances provided by capacitors C1 and C2 also change. At any instant in time, the capacitances provided by capacitors C1 and C2 are indicative of the instantaneous differential pressure between chambers 110 and 112. Known electrical circuits (e.g., a "tank" circuit characterized by a resonant frequency that is a function of the capacitance provided by capacitor C1 or C2) may be used to measure the capacitance provided by capacitors C1 and C2 and to provide an electrical signal representative of the differential pressure.
Some mechanical details of transducer assembly 100 will now be discussed. FIG. 2A shows a top view of housing 140 and FIG. 2B shows a sectional view of housing 140 taken in the direction indicated by the line 2B--2B as shown in FIG. 2A. Referring to FIGS. 1B, 2A, and 2B, it can be seen that housing 140 has a tubular configuration, and the walls of housing 140 are generally L-shaped. The L-shaped walls of housing 140 include a vertically extending portion 141 (defining the vertical portion of the L-shape) and a horizontally extending portion 142 (defining the horizontal portion of the L-shape). The horizontally extending portion 142 defines an annular upper surface 143 that acts as first support shoulder. The upper surface of the vertically extending portion 141 is stepped so that the housing 140 also defines a second annular support shoulder 144. Housing 140 also defines a central aperture, or through hole, 145 and an annular bottom support surface 146.
Referring again to FIGS. 1A and 1B, the upper cover 150 is circular. When transducer 100 is assembled, the outer periphery of the lower surface of upper cover 150 rests on the second support shoulder 144 of the housing 140. The upper cover 150 and the housing 144 are joined (normally by welding) to form an air tight seal between the two parts.
The lower cover 160 is also circular. Lower cover 160 defines an annular upper surface 166 that extends around the outer periphery of lower cover 160. The housing 140, diaphragm 120, and lower cover 160 are joined (normally by welding) together (1) so that the outer periphery of the diaphragm is trapped between the lower surface 146 of housing 140 and the upper surface 166 of lower cover 160; (2) so that an air tight seal is formed between diaphragm 120 and the lower cover 160, thereby sealing interior chamber 112; and (3) so that an air tight seal is formed between diaphragm 120 and housing 140, thereby sealing interior chamber 110.
Electrode 130 is fabricated from a rugged, rigid, electrically insulating (or nonconductive), material. Normally, electrode 130 is fabricated from a ceramic such as Alumina or Fosterite. Electrode 130 has a generally cylindrical shape. FIG. 3A shows a side view of electrode 130. FIG. 3B shows a bottom view of electrode 130 taken in the direction indicated by arrow 3B--3B as shown in FIG. 3A. Electrode 130 includes a relatively thin cylindrical outer section 134 and a thicker cylindrical central section 135. The outer section 134 defines an annular lower surface, or shoulder, 136 that extends around the outer periphery of the electrode 130. The central section 135 defines the planar, circular, lower surface 133 and the two conductors 131, 132 are disposed on the lower surface 133. As shown in FIGS. 1A, 1B, and 3A, conductor 131 includes a portion 131A that extends through a plated-through hole and connects to an upper surface of electrode 130. Similarly, conductor 132 includes a portion 132A that extends through another plated-through hole and connects to the upper surface of electrode 130. Electrode 130 is normally positioned within chamber 110 so that the space between conductors 131, 132 and diaphragm 120 is relatively small (e.g., on the order of 0.006 inches). The electrode 130 is also positioned so that the conductors 131, 132 are disposed in a plane that is parallel to the plane defined by the diaphragm 120 when the pressures in chambers 110, 112 are equal.
An aperture, or through hole, 137 extends entirely through the central portion 135 of electrode 130 at a location between the two conductors 131, 132. Aperture 137 permits the pressure of a fluid in inlet 153 to be applied to the upper surface of the diaphragm.
Housing 140 supports electrode 130 within the chamber 110. The lower shoulder 136 of electrode 130 essentially rests on the shoulder 143 defined by the horizontally extending portion 142 of housing 140.
Transducer assembly 100 also includes a wave washer 170 (i.e., a metallic, annular, washer that has been bent in one or more places in directions perpendicular to the plane of the ring) and a force transfer member 172. The wave washer 170 is disposed between the upper cover 150 and the transfer member 172, and the transfer member 172 is disposed between the wave washer 170 and the electrode 130. When the upper cover 150 is attached to the housing 140 (as shown in FIG. 1A), the wave washer 170 is sufficiently compressed to generate about 100 to 200 pounds of force. This force is communicated via the transfer member 172 to the electrode 130 and pushes the electrode down towards the diaphragm 120. The shoulder 143 of housing 140 supports the electrode 130 and resists movement of the electrode 130 towards the diaphragm 120. The large force generated by the wave washer essentially pushes the electrode onto the housing and holds the electrode 130 securely in place with respect to the housing 140.
During normal operating conditions, the diaphragm 120 moves or flexes within the chambers 110, 112 and does not contact electrode 130. However, when the pressure in chamber 112 becomes excessive, the diaphragm may contact and push against electrode 130. This condition is commonly referred to as an "over pressure condition". The relatively large spring force generated by the compressed wave washer 170 prevents the electrode 130 from moving, or becoming unseated, during over pressure conditions.
Transducer assembly 100 also includes one or more shims 174 disposed between the lower surface 136 of electrode 130 and the shoulder 143 of housing 140, and also between the upper surface of the electrode 130 and the transfer member 172. These shims are normally included to adjust the spacing between the conductors 131, 132 and the diaphragm 120. FIG. 4A shows a top view of one of the shims 174. FIG. 4B shows a sectional view of shim 174 taken in the direction indicated by line 4B--4B as shown in FIG. 4A. FIG. 4C shows a perspective view of shim 174. As shown, the shim 174 is characterized by a relatively thin annular (or "flat washer") shape. Shim 174 is further characterized by the following parameters: an outer diameter D, a thickness T, and a cross-sectional width W. In the case of shim 174, the cross-sectional width W is equal to the difference between the radius R1 (i.e., the distance between the center and the outer perimeter of the shim) and the radius R2 (i.e., the distance between the center and the inner perimeter of the shim). A typical prior art shim has an outer diameter D equal to 1.8 inches, a thickness T equal to 0.006 inches, and a cross-sectional width W equal to 0.08 inches.
Referring again to FIGS. 1A and 1B, transducer assembly 100 also includes two conductive pins 180, 182 and two conductive springs 184, 186. The conductive pins 180 and 182 extend through upper cover 150 and are electrically insulated from cover 150 by insulating plugs (e.g., melted glass plugs) 187 and 188, respectively. One end of conductive spring 184 physically and electrically contacts conductor 131A (and therefore electrically couples to conductor 131). The other end of conductive spring 184 is mechanically and electrically connected to pin 180. Similarly, one end of conductive spring 186 physically and electrically contacts conductor 132A (and therefore electrically couples to conductor 132). The other end of conductive spring 186 is mechanically and electrically connected to pin 182. This arrangement permits external circuits outside of transducer assembly 100 to electrically couple to conductors 131 (via pin 180 and spring 184) and 132 (via pin 182 and spring 186). Since diaphragm 120 and housing 140 are both conductive (and since they are welded together), external circuits may electrically couple to diaphragm 120 simply by coupling to housing 140. In practice, the body of transducer assembly 100 is normally grounded (thereby grounding diaphragm 120), so the capacitance provided by capacitors C1 and C2 may be measured simply by electrically coupling external measuring circuits to pins 180, 182.
In operation, transducer assembly 100 is normally used as an absolute pressure transducer. In this form, chamber 110 is normally first evacuated by applying a vacuum pump (not shown) to pressure tube 152. After chamber 110 has been evacuated, tube 152 is then sealed, or "pinched off" to maintain the vacuum in chamber 110. This creates a "reference" pressure in chamber 110. Although a vacuum is a convenient reference pressure, other reference pressures can be used. After the reference pressure has been established in chamber 110, the pressure tube 162 is then connected to a source of fluid (not shown) to permit measurement of the pressure of that fluid. Coupling the pressure tube 162 in this fashion delivers the fluid, the pressure of which is to be measured, to chamber 112 (and to the lower surface of diaphragm 120). The center of diaphragm 120 moves or flexes up or down in response to the differential pressure between chambers 110 and 112 thereby changing the capacitance of capacitors C1 and C2. Since the instantaneous capacitance of capacitors C1 and C2 is indicative of the position of the diaphragm 120, transducer assembly 100 permits measurement of the pressure in chamber 112 relative to the known pressure in chamber 110.
Transducer assembly 100 can of course also be used as a differential pressure transducer. In this form, pressure tube 152 is connected to a first source of fluid (not shown) and pressure tube 162 is connected to a second source of fluid (not shown). Transducer assembly 100 then permits measurement of the difference between the pressures of the two fluids.
For transducer assembly 100 to operate properly, it is important to maintain a stable geometric relationship between the various components of the assembly, and in particular between the diaphragm 120 and the conductors 131, 132. In operation of transducer assembly 100, the diaphragm 120 of course moves up and down (in response to changes in differential pressure) thereby changing the spacing between diaphragm 120 and the conductors. However, for transducer assembly 100 to provide consistently accurate readings, it is important to insure that the geometric relationship between the diaphragm 120 and the conductors 131, 132 remains constant over a long period of time for any particular differential pressure. Also, when manufacturing large numbers of transducer assemblies, it is important to provide the same geometric relationships in each assembly. By way of example, important characteristics of this geometric relationship include (1) the axial distance between the conductors and the diaphragm for any particular pressure; (2) the shape of the electrode 130 (e.g., tilting or warping of the electrode 130 will affect the spatial relationship between the diaphragm and the conductors); and the amount of tension on the diaphragm. For convenience of exposition, the geometric relationships between the various parts of the transducer assembly shall be referred to herein simply as the "geometric configuration". One problem with prior art transducer assembly 100 has been maintaining a stable geometric configuration.
Prior art transducer assembly 100 relies principally upon wave washer 170 and shims 174 to maintain a stable, desired, geometric configuration. Shims 174 are used to tune or adjust the distance between the diaphragm 120 and the conductors when the assembly 100 is assembled. After the transducer assembly is assembled, the large force generated by the wave washer 170 holds electrode 130 in a fixed position and thereby attempts to maintain a constant geometric configuration.
Although transducer assembly 100 holds the electrode 130 securely, the geometric configuration can vary by small amounts over time in response to, for example, temperature changes. The thermal coefficient of expansion for the metallic housing 140 is typically larger than the thermal coefficient of expansion for the ceramic electrode 130. Therefore, heating or cooling the transducer assembly can generate shear forces within the assembly that affect the geometric configuration. Mechanical stress can build up in the assembly in response to heating or cooling, and when the stress gets large enough, the electrode and the housing will move relative to one another to release the stress. This type of movement is sometimes referred to as "stickslip" or "mechanical hysteresis". These stick-slip movements affect the geometric configuration and can adversely affect the accuracy of the transducer assembly 100.
The performance of transducer assembly 100 may be characterized by at least two parameters. The first parameter is "stability", and the second parameter is "repeatability". Since the geometric configuration of transducer assembly 100 can change over time in response to heating or cooling, the transducer assembly 100 is susceptible to poor "stability". Both heating and cooling can cause changes in the geometric configuration. However, cycles of heating and cooling will not generally produce corresponding, repeatable, shifts in the geometric configuration. For example, if a transducer assembly at an original temperature, characterized by an original geometric configuration, is heated enough to cause a shift in the geometric configuration, and the assembly is then cooled back to the original temperature, the geometric configuration will not generally return to the original configuration. Rather, the assembly will normally now be characterized by a new geometric configuration. In other words, the variation in the geometric configuration caused by temperature shifts is characterized by mechanical hysteresis. This mechanical hysteresis is in general caused by non-conservative effects such as friction. Since changes in the geometric configuration are characterized by this hysteresis, the transducer assembly has relatively poor "repeatability".
U.S. Pat. No. 5,271,277 discloses one apparatus that attempts to maintain a stable geometric relationship between the conductor and the diaphragm over time. As shown by FIGS. 1-4 of that patent, in that apparatus the electrode is connected to the housing via an annular support member. One end of the annular support member is welded to a flexible portion of the housing. The other end of the annular support member is bonded, such as by glass frete bonding material, to the electrode. Rather than using a large spring force (e.g., such as that generated by a wave washer) to push the electrode securely against the housing, this apparatus uses the annular support member to isolate the electrode from the housing. In fact, the annular support member provides the only support for the electrode. This apparatus tends to isolate the electrode from some of the thermal expansion forces generated within the housing. However, this apparatus is complicated to build and lacks ruggedness. At least one problem with this apparatus relates to the glass frete bond between the annular support member and the electrode. Such bonds are difficult to fabricate and are also relatively fragile.
U.S. Pat. No. 4,823,603 discloses another apparatus for maintaining stable geometric relationships over time. As shown by FIGS. 10-15 of that patent, in that apparatus roller bearings are disposed between the electrode and the housing. A spring force is used to prevent the electrode from moving in a direction perpendicular to the diaphragm and the roller bearings permit the electrode to move in a direction parallel to the diaphragm. The movement permitted by the roller bearings prevents thermal stress from warping the electrode or from causing a "stick slip" type movement. In this apparatus, it is difficult to use large spring forces, so a relatively small spring force is typically used. The small spring force renders the apparatus unable to tolerate a large over pressure condition. That is, a relatively large over pressure condition can unseat the electrode and thereby alter the geometric configuration.
It is an object of the present invention to provide a pressure transducer assembly with an improved mounting for the electrode.